Hello,
This seems to be totally overengineered. Having MTA a provide sendmail
which uses MTA b for remote deliveries is no common usage scenario on
which any effort should be spent in the Debian packaging
infrastructure.
Actually the only sane explanation for wanting to install two MTAs I
ever heard of was "I am running X now and want to switch to Y. - I'd like
to test Y in the real system before going live."

I know of at least one firewall product that includes 2 copies of sendmail,
one for accepting messages from the Internet and one for processing and
sending them to the internal servers.

This is actually a common setup when using amavis-ng, spamassasin and
the rest of the spam fighting packages current available in Debian. It
is currently difficult to provide the full capabilities of what all of
these packages can do together unless you have 2 systems upon which to
spread the 2 MTAs. The 'front' and 'back' MTAs in such an antispam
configuration have different requirments, and they can best be met by
using different MTAs for each.
I think it is worth a small amount of effort to allow multiple MTAs to
co-exist.
Stuart
Stuart R. Anderson anderson@netsweng.com
Network & Software Engineering http://www.netsweng.com/
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